1. Field of the Invention
The invention generally relates to tunable lasers and in particular to tunable lasers for use in a dense wavelength division multiplexers (DWDM).
2. Description of the Related Art
A DWDM is a device for simultaneously transmitting a set of discrete information channels over a single fiber optic transmission line. A conventional fiber optic transmission line is capable of reliably transmitting signals within a bandwidth of 1280 to 1625 nanometers (nm), the xe2x80x9clow lossxe2x80x9d region for silica fiber. Within that overall bandwidth, the International Telecommunications Union (ITU) has defined various transmission bands and specified certain transmission channel protocols for use within each transmission band. One example of a transmission band is the ITU xe2x80x9cCxe2x80x9d band, which extends 40 nm from 1565 nm to 1565 nm. Within the C band, specific transmission channel protocols of 40, 80, or 160 discrete channels are defined and, for each protocol, the ITU has defined a grid of transmission wavelengths, with each line corresponding to an acceptable transmission wavelength. For the 40 channel protocol, the corresponding ITU grid has 40 lines with channel spacing of 0.8 nm; for the 80 channel protocol, the corresponding ITU grid has 80 lines with channel spacing of 0.4 nm; and so forth. The protocols have been defined to ensure that all DWDM transmission and reception equipment is fabricated to operate at the same wavelengths. Other exemplary ITU transmission bands are the S- and L-bands.
To simultaneously transmit the set of channels on a fiber optic cable, the DWDM employs a set of the individual lasers, with one laser per channel. FIG. 1 illustrates a DWDM 100 having forty individual lasers 102 for transmitting optical signals via a single optic fiber 104. An optical multiplexer 106 couples signals received from the individual lasers via a set of intermediate optic fibers 107 into output optic fiber 104. Each laser transmits at a different wavelength of the forty channel ITU C band. This enables forty separate channels of information to be transmitted via the single optical fiber 104 to a de-multiplexer (not shown) provided at the far end of the optical fiber.
To permit the DWDM to transmit forty separate channels simultaneously, each individual laser must be tuned to a single ITU transmission channel wavelength. Preferably, widely tunable lasers (WTLs) are employed as the transmission lasers to permit any of the individual lasers to be tuned to any of the ITU channels. The wide tunability is achieved by choosing the amplifying medium with a wide gain curve (typically a semiconductor structure) and configuring the WTL in such a way that low cavity loss is only achieved at a narrow wavelength region. The WTL is tuned by changing the aforementioned peak in the spectral response of the cavity to operate at a particular resonant wavelength aligned with a selected one of the ITU channel wavelengths. The desire to achieve low bit error rate leads to requirements for high spectral purity of the emitted light which is usually stated in terms of the side mode suppression, i.e. the ratio of the light intensity emitted by the WTL at the side band wavelength to the intensity of the main mode. While the requirements may differ from system to system, typically for reliable operation of a DWDM transmitting ITU channels, the side mode suppression should be 30 decibels (dB) or better. Such requirements often are not easily reconciled with the wide tunability, particularly with relatively inexpensive WTLs. Furthermore, while monolithic semiconductor WTLs, currently the state of the art devices, provide the necessary degree of miniaturization and ruggedness, tuning of such devices involves a complicated tuning pattern. That is, tuning currents are applied to multiple sections of the laser and their influence on the emitted wavelength and spectral purity is not independent.
Accordingly, it would be desirable to provide an improved WTL capable of being more easily, precisely and reliably tuned to a selected wavelength and in particular to provide an improved WTL capable of meeting the aforementioned 30 dB sideband requirement for use within DWDMs transmitting at ITU channel wavelengths. It is to these ends that the invention is primarily directed.
In accordance with the invention, a semiconductor laser is provided having a cavity comprising a gain chip, an interferometric wide tuning port, and a reflective etalon. The gain chip provides radiant energy and amplification for resonance within the cavity. The reflective etalon, which may be reflective Fabry-Perot etalon, limits resonance within the cavity to a set of sharp resonance peaks. The interferometric wide tuning port, which may be a Mach-Zehnder interferometer, selects one resonance peak out of the aforementioned set of peaks based on a wide tuning profile having a broad peak. The wide tuning port is configured to align a wavelength of the broad peak of the wide tuning profile at a selected wavelength. The reflective etalon is configured to align a wavelength of one of the sharp peaks also at the selected wavelength. In this manner, the laser cavity resonates primarily at only the selected emission wavelength. Other sharp resonant peaks permitted by the reflective etalon are filtered by the wide tuning port to substantially limit the amplitudes thereof. Hence, transmission sidebands of the laser are substantially reduced. With appropriate selection of components, the side bands may be limited to amplitudes 30 dB below the selected transmission wavelength.
In an exemplary embodiment, the semiconductor laser is used as a WTL within a DWDM transmitting at ITU channel wavelengths. A Mach-Zehnder wide tuning port and a reflective Fabry-Perot etalon are formed together on a single silicon tuning chip coupled to a separate gain chip with an output optical channel of the gain chip coupled to an input optical channel of the wide tuning port. The wide tuning port includes a splitter for splitting the input channel into a pair of channels of differing lengths and a combining section for combining the pair of optical channels into a single output channel. The single output channel of the wide tuning port is coupled to a main trunk channel of the reflective etalon. A Fabry-Perot waveguide side channel having reflecting surfaces at opposing ends is formed adjacent the main trunk. A central portion of the Fabry-Perot side channel is sufficiently close to the main trunk to permit evanescent coupling of radiant energy propagating therein. Hence, radiant energy entering the reflective etalon from the wide tuning port is coupled into the Fabry-Perot side channel, where it is reflected back and forth between the reflecting surfaces. In turn, the radiant energy reflecting within the Fabry-Perot side channel is coupled back into the main trunk channel also via evanescent coupling. At resonance wavelengths of the Fabry-Perot side channel, substantially all of the radiant energy coupled back into the main trunk propagates toward the wide tuning port. At other wavelengths, the fraction of the energy returning to the gain chip is greatly diminished.
Thus, with this configuration, optical signals generated by the gain chip propagate through the wide tuning port and into the reflective etalon. Optical signals at the resonance wavelengths of the Fabry-Perot waveguide side channel are then reflected back through the wide tuning port to the gain chip then are reflected again by the gain chip. Optical signals not at the resonance wavelengths of the Fabry-Perot side channel do not couple effectively into the reflective etalon. In this manner, resonance within the laser cavity is permitted only at the various resonance wavelengths of the side channel, resulting in the aforementioned sharp resonance peaks. The resonance wavelengths of the side channel depend primarily upon the length of the side channel. The reflective etalon is heated, or otherwise controlled, to vary the length of the side channel so as to align and maintain one of the sharp resonance peaks at a selected transmission wavelength, such as at a particular ITU channel wavelength.
Hence, optical signals at the resonance wavelengths of the Fabry-Perot waveguide side channel reflect back and forth between the gain chip and the reflective etalon through the Mach-Zehnder wide tuning port. As the optical signals propagate through the wide tuning port, the signals are split between two channels of differing lengths resulting in optical interference. The optical interference limits the ability of the laser cavity to resonate at wavelengths wherein destructive interference occurs. As a result, resonance within the overall laser cavity is substantially limited to within a broad peak determined by the relative lengths of the two channels. The wide tuning port is heated, or otherwise controlled, to vary the relative lengths or refractive indexes of the two channels so as to align and maintain the broad peak at the selected transmission wavelength. In this manner, only one of the sharp resonant peaks permitted by the reflective etalon is also aligned with the broad peak permitted by the wide tuning port. Other sharp resonant peaks permitted by the reflective etalon, representing sidebands, are substantially reduced by the interference provided by the wide tuning port or are at wavelengths otherwise not generated by the gain chip. Hence, the laser cavity resonates substantially only at a single wavelength, namely the selected transmission wavelength. The amplitudes of sidebands are thereby greatly limited. Resonance within the laser cavity generates coherent light which is output through an opposing end of the gain chip for use as a transmission signal. The signal may be modulated to encode digital information. By limiting the amplitudes of the sidebands to, for example, 30 dB below the amplitude of the selected transmission wavelength, a data transmission error rate can be reduced to as low as 10xe2x88x929 or below.
Thus, the invention provides an improved WTL capable of being easily, precisely and reliably tuned to a selected wavelength while also being capable of meeting the aforementioned 30 dB sideband requirement for use within DWDMs transmitting at ITU channel wavelengths. Other embodiments, features and advantages of the invention will be discussed below.